Impervious Cover TMDL Methodology for Connecticut. Chris Bellucci Connecticut Department of Environmental Protection [email protected] Biological Condition. % Impervious Cover. Impervious Cover …. Why now ?
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Strength of Evidence – Scientific Literature
Adapted from 2003 Center for Watershed Protection Research Monograph Number 1
“CTDEP developed a TMDL support document that provides a scientific basis that IC is an appropriate surrogate measure of impacts caused by stormwater (i.e. "other appropriate measure") and aquatic life use assessments using macroinvertebrates (i.e. "biomonitoring approach") provide an appropriate endpoint to measure progress of implementation.” From Draft Eagleville Bk TMDL
% Impervious Cover and Macroinvertebrates
% of Reference Community compares 7 metrics- Taxa Richness, Modified HBI , Scraper/Filterer, EPT/Chironomidae, % Dominant Taxa, EPT Index, Community Loss
% Impervious Cover
Percent Impervious Cover and
Aquatic Life Use Support (ALUS)
Red is Urban Areas Under MS4
Flow Alteration, Agriculture,
e.g. IC upstream of Impaired ALUS segment is 20%
Assume not enough info to separate WLA and LA
TMDL Target = 11% IC
WLA = (20% - 11% / 20%) x 100 = 45% Reduction in IC
LA = 45% Reduction in IC
MOS = 1% (12% Threshold - 11% Target)
The IC TMDL target is not intended to be used as the basis for permit limits, or a "not to be exceeded" limit on development, or a measurement of compliance with Water Quality Standards.
Biological assessment determines ALUS not met
303 (d) Listed ALUS-Cause Unknown
303 (d) Listed ALUS -
Cause = Stormwater
SI Investigation determines stormwater is most probable cause
IC TMDL Methodology
Can we offset Impacts of IC by BMPs and “Smart” Land Use Planning
(i.e. Will the bugs get better?)
Describe the variables that make some streams with high IC good and some streams with high IC bad
Need to reassess our aquatic life goals for streams with high IC
The Storm Water Management Model is a comprehensive computer model for analysis of quantity and quality problems associated with urban runoff. Both single-event and continuous simulation can be performed on catchments having storm sewers, or combined sewers and natural drainage, for prediction of flows, stages and pollutant concentrations. Extran Block solves complete dynamic flow routing equations (St. Venant equations) for accurate simulation of backwater, looped connections, surcharging, and pressure flow. Modeler can simulate all aspects of the urban hydrologic and quality cycles, including rainfall, snow melt, surface and subsurface runoff, flow routing through drainage network, storage and treatment. Statistical analyses can be performed on long-term precipitation data and on output from continuous simulation. The model can be used for planning and design. Planning mode is used for an overall assessment of urban runoff problem or proposed abatement options.
Recommendation - Stormwater Management
When it rains, stormwater runoff carries pollutants to streams, alters natural streamflow, and causes negative impacts to fish and bugs that live in the stream. The greater the IC, the greater the chance of killing or reducing numbers of sensitive fish and bugs.
Recommendation - Stormwater Management
“Pluralitas non est ponenda sine necessitas”
Plurality should not be posited without necessity
"when you have two competing theories which make exactly the same predictions, the one that is simpler is the better."
William of Ockham (c.1285–1349)
Wireless Rechargeable Razor-
“For CT, IC model is mo’ better than competing stormwater models for TMDL Development”